A Bright Spot in a Dark Sky

Energy is strange stuff. You can't see it or hold it. You can't measure it directly in the way that you could measure distance, time, mass, etc. (one could argue that you can't measure anything directly, but we'll not go down that rabbit hole). In addition to its elusive nature, another weird thing about energy is that it is differences that make it useful, not necessarily the amount.

Let's consider how we might get energy from a bunch of spheres.

1. We could drop the spheres in a gravitational field. As the spheres fall we could have them collide with a generator and extract energy...but this relies on being able to let them fall . If the spheres are confined to a level surface, even if they are in a strong gravitational field (say in the vicinity of a black hole), there would be no way of using the gravitational potential energy.
2. Another possibility would be to have the spheres rolling horizontal along a surface. Again we could collide them with a generator and get usable energy, but this relies on the spheres moving at a different speed than the generator. If the generator is moving with the spheres, there would be no collision and no energy extracted.
3. If the spheres were hot, we could potentially harness the thermal energy. But the only way for that to happen would be for heat to flow, which would require something colder than the spheres (i.e. things at different energies).
4. The spheres could be electrically charged to 1,000,000 volts, but without connecting the spheres to something at a different voltage, no electrical current would be possible.

In principle you could have an infinite store of energy and not be able to use one bit of it. It's often said that the sun is the primary source of energy for the earth, which it is of course. But what is equally important is that the rest of our surroundings are cold. What makes the sun useful is that it is a hot spot in an otherwise cold sky.